1. Field of the invention
The present invention relates to a process for preparing a toner used in image forming processes such as electrophotography and electrostatic recording.
2. Related Background Art
As shown in FIG. 3, toners are commonly produced by melt-kneading a thermoplastic resin and a colorant as exemplified by a dye, a pigment or a magnetic powder to form a uniform dispersion, followed by the step of pulverizing a cooled kneaded product and optionally the step of classifying the pulverized product, to prepare a fine powder, i.e., toner particles, with a given particle diameter and particle size distribution. The toner particles are further mixed with an external additive such as an inorganic fine powder or an organic fine powder in order to improve properties such as fluidity, charge stability, lubricity and cleaning performance of toners so that stable image reproduction can be continued through the step of development, the step of transfer and the step of cleaning to remove untransferred toner from a photosensitive member, in copying machines. Toners are thus produced.
In these steps, however, coarse particles, melt-adhered coarse particles due to mechanical heat generation and reagglomerates caused by van der Waals force may be produced to cause clogging of fine gaps in a developing machine, or to cause various faulty images because of their presence as defectively charged particles. In order to remove such coarse particles and reagglomerates, the toner particles are passed through a sieve with an aperture size of, for example, 100 to 250 .mu.m. An apparatus having a sieve includes, for example, a multiple Gyro sifter (trade name for a gyratory sifter), and its vibration method includes mechanical vibration and ultrasonic vibration.
Coarse particles and reagglomerates can be first removed by such a method. Under existing circumstances, however, there is room for some improvement in production stability and toner quality. For example, as a result of repeated use of sieves for a long period of time, wires that form a wire cloth may wear because of friction between the toner particles and the wire cloth of the sieve to make the aperture size large or to cause a break of wires to make it impossible to achieve what is intended. In particular, in the case of magnetic toners containing magnetic powder in toner particles or toners comprised of toner particles mixed with a very hard and highly abrasive inorganic fine powder or organic fine powder, the wires may wear great. In addition, in recent years, toner particles are apt to have a smaller particle size for the pursuit of more highly detailed image quality, where an increase in specific surface area of toner particles per unit weight results in an increase in the frequency of their contact with the wires of the wire cloth. If the diameter of the wires is made smaller in order to make aperture size smaller so that the coarse particles and reagglomerates can be effectively removed, the problem arises that wear of wires is hastened.
In order for toner particles and external additives to be effectively passed through sieves, the sieves are vibrated in various ways.
When, however, a conventional sieve with a wire cloth of plain weave is vibrated to allow toners to pass therethrough, the surfaces of toner particles tend to be deformed to cause changes in the state of dispersion of external additives in toners or the state of adhesion between external additives and toner particles, resulting in changes in charge performance and powder characteristics of toners to often cause a lowering of the properties of toners.
Hence, it is earnestly sought to provide a method by which toners can be efficiently treated after the step of external addition, without causing any lowering of the properties of toners.